1. Field of the Invention
The present invention generally relates to motion picture film registration techniques and, more particularly, to a system and method for correctly registering frames of motion picture film during the projection process to provide enhanced resolution of the projected images.
2. Description of the Related Art
Film projectors and the motion pictures they project have amazed and delighted audiences around the world for over a century. However, the term xe2x80x9cmotion picturesxe2x80x9d is really a description of an illusion, for the pictures do not actually move at all. To the contrary, still images, typically 24 per second, must be projected in as static a manner as possible. This is not a simple proposition given the fact that in one minute 1,440 images or xe2x80x9cpicturesxe2x80x9d are presented to the viewer. The illusion of motion is created by the differences between succeeding still images and is dependent upon the precise positioning or xe2x80x9cregistrationxe2x80x9d of these images with respect to an aperture in the projector.
One of the problems associated with existing motion picture film projectors occurs when succeeding images are positioned in slightly different locations in the projector""s aperture. When this happens, a given point on those images will appear to be blurred, even if individual images show that point to be perfectly sharp. Of course, individual images in motion pictures are not viewed separately, but, rather, over time in rapid succession.
In the context of motion picture technology, the word xe2x80x9cregisterxe2x80x9d (the root for xe2x80x9cregistrationxe2x80x9d) is not formally defined. Perhaps the closest applicable general definition is provided in the Random House Dictionary Of The English Language, Second Edition Unabridged (1983): xe2x80x9c(11b) Print. Correct relation or exact superimposition, as of colors in color printing.xe2x80x9d In the art of motion pictures, however, xe2x80x9cregistrationxe2x80x9d has a slightly different meaning. Motion picture photographic xe2x80x9cregistrationxe2x80x9d means the repeated placement of each image, one after another, in as precise a manner as possible throughout the entire chain that comprises the system of cinema imaging. As described below, there are several steps in this chain, starting with creation of the image and ending with its projection.
At the outset, film is moved through the camera intermittently and positioned by xe2x80x9cregistration pinsxe2x80x9d in precisely the same place, called the xe2x80x9caperturexe2x80x9d. In this way, a succession of areas called xe2x80x9cframesxe2x80x9d are exposed to light, thus creating xe2x80x9clatent imagesxe2x80x9d that become a visible image after development. Because the xe2x80x9cframexe2x80x9d defines the rectangular space on the film that is occupied by the xe2x80x9cimage,xe2x80x9d the terms xe2x80x9cframexe2x80x9d and xe2x80x9cimagexe2x80x9d shall be used interchangeably and synonymously with each other herein.
During editing and other post-production processes, images created in the camera are modified when appropriate. Then they are duplicated for distribution through a process that involves intermediate steps. These intermediate steps include contact printing of an inter-positive (xe2x80x9cIPxe2x80x9d), and using that IP to make inter-negatives (xe2x80x9cINxe2x80x9d).
In the next step, the IN is transferred onto release print film via high-speed contact printing, which is an unregistered process that operates at up to 17xc3x97-play speed, or faster. The xe2x80x9crelease printsxe2x80x9d made by this process are distributed to theaters for projection. When the release prints are projected, the frames are intermittently positioned in a fixed position relative to the xe2x80x9caperturexe2x80x9d of a projector""s xe2x80x9cgate.xe2x80x9d Light from a lamphouse in the projector projects the images onto a screen for viewing by an audience.
Registration is not a factor in creating high resolution still photographic images. The photographer and viewer of a still photograph are concerned only with a single image recorded and printed from a single piece of film containing that image. However, registration is a crucial component required for high resolution imaging in motion pictures. As noted above, motion pictures are dependent on thousands of images seen one after another in rapid succession. Thus, in motion pictures, the collective impression of resolution or sharpness is highly dependent upon the repeatable, accurate positioning in the projector of every image that is photographed and projected.
xe2x80x9cResolutionxe2x80x9d is another term for sharpness or clarity. In motion pictures, resolution is a function of several factors, which include: (1) lens sharpness; (2) film negative granularity; (3) repeatable, accurate registration of the film in the camera""s aperture; (4) repeatable, accurate registration during exposure of film IP""s, IN""s, and release prints; (5) film print granularity; and (6) repeatable, accurate registration of the release print in the projector. Of all these factors, nos. 4 and 6 are the most severely flawed in current motion picture technology. According, a long-felt need has existed for a system and method that can address these factors and thereby provide more precise registration and enhanced resolution across the entire system of motion picture imaging.
The final measurement of motion picture resolution must be made by analysis of a projected image at 24 frames per second (standard) or faster, not by inspection of individual frames as with still photography. Ideally, the registration precision of the projector should match that of the camera. Unfortunately, that is not now and never has been the case. As noted above, motion pictures cameras use highly precise, mechanically activated xe2x80x9cregistration pinsxe2x80x9d to achieve and maintain final and repeatable film positioning from frame to frame. On the other hand, theatrical projectors use registration techniques that are, at best, considerably inaccurate about both the longitudinal and lateral axes. This inaccuracy gets progressively worse as various mechanical parts in the projector""s intermittent movement and gate are subjected to normal wear over time. But the primary cause of this inaccuracy is the 100-year-old design of the projector movement itself. Projector gates, intermittent sprockets, and the xe2x80x9cGenevaxe2x80x9d mechanism that turns these sprockets in a pause-then-rotate cadence have failed to evolve in any meaningful manner.
As described more fully below, projector registration in its present state is primarily achieved by means of the friction provided by spring tension in the projector gate, acting against the film, which is advanced by the rotational movement of the intermittent sprocket. This intermittent sprocket is typically positioned about 2 to 4 inches or more below the aperture and pulls the film through the gate. When the intermittent sprocket stops pulling the film, the spring tension in the gate acts on the film sandwiched within it and friction causes the film to stop. But this is a highly passive design that lends itself to imprecision. For example, gate friction varies due to adjustable spring tension. Moreover, the film print itself exhibits variable xe2x80x9cslicknessxe2x80x9d due to waxing, wear and other environmental factors. Therefore, each succeeding frame simply cannot be registered in the exact same location as the preceding frame.
During projection, inaccurate longitudinal registration of the film produces an up and down film movement called xe2x80x9cjitter,xe2x80x9d while inaccurate lateral registration produces a side-to-side film movement called xe2x80x9cweave.xe2x80x9d Both jitter and weave are greatly magnified by the extreme enlargement of projection. At a minimum, jitter and weave in any noticeable amount will result in a softening and blurring of detail and impair the resolution of the projected images.
The use of larger images on film, as with the various 70 mm formats, creates a sharper image on screen simply because less enlargement is required to fill the screen. Consequently, the jitter and weave of the image is less noticeable with 70 mm release prints. However, the higher cost and the lack of 70 mm projectors in most theaters renders the various 70 mm formats moot as an option, except in a few xe2x80x9cspecial venuexe2x80x9d theaters. Indeed, if jitter and weave in the 70 mm projectors could be reduced or eliminated, the projected image would be even sharper.
Current theatrical motion picture projectors inherently create jitter and weave because they lack any kind of positive film registration technology. Furthermore, the high-speed printing process used to manufacture most IP""s and IN""s and thousands of release prints distributed to theaters creates yet another level of jitter and weave in itself. In order to understand the problems that this causes when the film is finally projected, it is necessary to understand how motion picture images are photographed.
In a typical camera movement, film is driven from the camera magazine by a constant speed sprocket, which maintains an upper loop of film. A pulldown claw driven by an eccentric cam-like movement penetrates the film""s perforations and pulls the film into precise registration in the camera""s aperture. The upper loop, much of which is taken up for this operation, is replenished by the continuous rotation of the constant speed sprocket. Next, with the film stopped, the registration pins penetrate adjoining perforations in the film, while simultaneously the pulldown claw retracts and begins moving back into position to take hold of the next length of film to be pulled down. Meanwhile, as the registration pins penetrate the film""s perforations, their tapered teeth gently move the film into precise position. The registration pins are restricted to a simple back and forth movement and are locked in a predictable, repeatable accurate mechanical design. Also, the registration pins are located immediately adjacent to and often surrounding the portion of the film to be exposed. For these reasons, they are very accurate and allow the camera to expose a continuous succession of images in precise registration.
In a typical projector movement, however, there is very little similarity to camera movements. Although a typical projector has a constant speed sprocket that feeds film to maintain an upper loop, all similarity to cameras ends at that point. Unlike a camera, the film""s advance into the projector gate is provided by a powerful pull from an intermittent sprocket positioned below the projector gate""s aperture, through which light is passed during projection. The projector gate is a gently curved, spring tensioned xe2x80x9ctrapxe2x80x9d that exerts friction on the film and thus acts in opposition to the motive force that pulls film through it. When the intermittent sprocket stops pulling the film through the gate, the friction exerted by the gate stops the film. Unfortunately, this mechanism cannot duplicate the precise registration provided by cameras for several reasons.
First, the gate""s friction, as applied to the movement of the film, is adjustable and varies from projector to projector. Higher friction provides heightened opposition to the film""s intermittent movement, but simultaneously demands that more torque be applied to overcome this static friction for frame-to-frame advance. This can cause film stretching or, in extreme cases, breakage. Also, gate friction that is too high may cause so much film resistance that the teeth of the intermittent sprocket will deform the perforations in the film during pulldown, which leads to deteriorating registration in every subsequent showing of the film. But if gate friction is reduced too much, the film may continue moving slightly after the rotation of the intermittent sprocket stops. In these cases, the film overshoots the proper location in a way that is prone to be erratic from frame-to-frame.
Second, film prints are often waxed or otherwise provided with a slick surface in order to slide through the gate with reduced resistance and/or to help prevent stretching of the film. This prevents the film from stopping in the gate in precisely the same place from one frame to the next.
Third, film often shrinks or expands due to age, humidity and other factors. Thus, the distance from the image in the aperture to the intermittent sprocket necessarily varies. The effect of such shrinkage or expansion increases with greater lengths of film. Therefore, the distance between the aperture and the intermittent sprocket includes further margin for error.
Fourth, the mechanism that advances the intermittent sprocket is driven by a xe2x80x9cGenevaxe2x80x9d movement, which is subject to wear. The Geneva movement is well known and comprises a xe2x80x9cMaltesexe2x80x9d cross-shaped device, with slots cut into each cross. A rotating cam-like device turns within it, with a pin that engages the slots in the cross. This produces a pause-then-rotate intermittent movement, which is then applied to a shaft connected to the intermittent sprocket that pulls the film through the aperture from below the gate. Although the Geneva movement turns in an oil bath that is designed to inhibit metal-to-metal contact, like any mechanical device, there is always some wear. This wear causes slight imprecision in the application of the motive power to the connecting shaft, the intermittent sprocket and the film itself, which is then magnified by the act of projection.
Fifth, the slightest bend in the shaft connecting the Geneva movement to the intermittent sprocket will impart an eccentric movement to the intermittent sprocket, so that instead of rotating in a circular movement, it will rotate in a slight oval-shaped pattern. This in turn exacerbates any imprecision in the Geneva movement, which, in turn, impairs resolution of the projected image.
Finally, release prints are made on non-registered, high-speed printers that introduce additional imprecision by placing the images in different positions with respect to the film edges and perforations. In other words, this non-registered printing process microscopically misplaces frames in such a way that they are no longer located in a precise, repeatable relationship to the edges and perforations of the film.
Certain special-purpose, pin-registered projectors have been built for a type of special effects cinematography called xe2x80x9cprocess shots,xe2x80x9d but these projectors were not designed for theatrical projection. Instead, they were designed to achieve precise registration with the assumption that they would be showing prints made on some type of relatively slow, highly accurate printer, rather than release prints duplicated by high-speed, non-registered contact printers.
Two new projector designs have recently appeared for 70 mm special-venue applications that provide some equivalent of pin registration. The Mega-Systems projector has two intermittent sprockets, placed both above and below the aperture. A curved, one-sided gate mechanism is brought into intimate contact with the film by sliding back toward it. This design is intended to allow for film shrinkage or expansion while still providing positive registration. The Linear Loop projector by IWERKS seeks to achieve the equivalent of positive pin-registration by using blasts of controlled, compressed air to advance film across a parallel set of linear sprockets placed on either side of the projector""s aperture. These linear sprockets, which are analogous to railroad tracks, hold the film""s perforations as succeeding frames are advanced by means of a xe2x80x9cstanding wavexe2x80x9d of film that rolls across the sprockets, propelled by the air blast. Though these two projectors provide registration that duplicates, somewhat, the positive registration found in cameras, they do not address the misalignment created by non-registered high-speed contact printers.
In another area of motion picture technology called xe2x80x9ctelecine,xe2x80x9d where motion picture images are transferred to videotape, various methods have been developed to achieve a stable image. While there are differences in the devices and methods used to achieve image stabilization in these telecine-based systems (sometimes called xe2x80x9celectronic pin registrationxe2x80x9d), they all have one thing in commonxe2x80x94they all seek to stabilize the film image by reference to the film edges and/or perforations on the film. This is acceptable in telecine, because telecine uses xe2x80x9clow contrastxe2x80x9d prints that are made at 180 feet per minute in xe2x80x9cwet gatexe2x80x9d contact printers. Thus, the image position on low contrast prints used in telecine bears a relatively accurate relationship to the film""s edges and/or perforations. However, the technique of using the edges and/or perforations on release prints as a reference to stabilize the film image wrongly assumes that the images on the film are correctly registered with respect to the edges and perforations, as they are in the camera or with prints made on relatively slow, highly accurate printers. As discussed above, theatrical release prints are made on non-registered contact printers at speeds often in excess of 1,500 feet per minute. This high-speed, non-registered printing process microscopically misplaces frames in such a way that they are no longer located in a precise, repeatable relationship to the edges and perforations of the film.
While various types of electronic pin registration and/or image stabilization methods and technologies work well when scanning prints made on registered printers, they cannot correct for improper placement of the image relative to the edges and/or perforations of the film. As discussed above, such improper placement is a common occurrence due to errors engendered in high-speed contact printing of theatrical release prints. Consequently, electronic pin registration has limited value for theatrical projection. In fact, none of these systems were designed with that purpose in mind. Rather, they all declare to be directed toward the process of scanning film to video or digital electronic form.
Accordingly, there has existed a definite need for a system and method that can achieve precise image stabilization and enhanced resolution for theatrical motion picture film projection, that corrects for misplacement of images on the release print film (compared to the original negative), and which does not rely on the edges of the film or its perforations to do so. The present invention satisfies these and other needs and provides further related advantages.
The present invention provides a system and method for precisely registering frames of film with respect to an aperture in a motion picture film projector. The projector comprises a gate for receiving and guiding film during intermittent advancement of the film through the projector. In accordance with the invention, information is applied to the film corresponding to the location of the frames on the film. The information associated with each frame is read by a sensor prior to projection to determine the location of the frame with respect to the aperture. If the frame is misregistered with respect to the aperture once the film stops in the gate, an actuator moves the film relative to the aperture to achieve the correct registration. By repeatedly registering the frames in the exact same location with respect to the aperture, the resolution of the projected motion picture image is substantially and advantageously enhanced.
In conventional projector designs, the gate is connected to the projector in a fixed manner relative to the aperture. In accordance with one embodiment of the present invention, however, the gate is adapted to move relative to the aperture. Movement of the gate relative to the aperture is provided by an actuator connected to the gate. In one form of the invention, the actuator comprises a piezoelectric motor or a moving coil motor. Both of these types of motors can be used in conjunction with a flexure stage to provide the required gate movement. The actuator preferably is configured to move the gate in increments as small as approximately 0.000002 inches in about 1 millisecond or less, depending on the frame rate. In addition, the actuator and the gate are configured such that the actuator can move the gate at least about 0.006 inches in both the X direction and the Y direction.
The information necessary to move the gate by the required amount is provided by the registration information applied to the film. In one form of the invention, the registration information comprises a registration reference mark that is capable of being read by a sensor. The registration reference mark preferably comprises a plurality of different shapes that are read by the sensor. In one aspect of the invention, the plurality of different shapes comprises at least a circle and a square or rectangle, where the diameter of the circle is equal to the width of the square or rectangle. In addition, a triangle may be positioned adjacent to the circle and the square or rectangle to provide further registration information.
The registration information is applied to the film in the same location relative to each frame. In this regard, the registration information preferably is located on the film in an area outside the frame, and, most preferably, the registration information is located in the space between adjacent frames. Furthermore, if desired, redundant registration information may be applied to the film for each frame.
The sensor that reads the registration information on the film may take a variety of forms. In one embodiment, the sensor comprises a light-based sensor. For example, the sensor may comprise an LED array on one side of the gate that transmits light through the registration reference mark on the film. The transmitted light is received by a CCD array on the other side of the gate. If desired, one or more mirrors may be used to reflect the light transmitted from the LED array onto the CCD array. In addition, redundant sensors may be used to read redundant registration information associated with each frame.
A registration processor controls the operation of the sensor and processes the registration information for each frame to determine the location of each frame with respect to the aperture. The location of these frames is determined by reading the registration information associated with each frame. The registration processor then uses the registration information to compare the location of a frame in the gate relative to the immediately preceding frame. If a frame is not properly registered with respect to the aperture in the same place as the immediately preceding frame, then the registration processor calculates the amount of film misregistration. Based on the amount of misregistration, the registration processor generates an appropriate output signal that is delivered to the actuator. This output signal commands the actuator to move the gate in such a manner that the frame is correctly registered relative to the immediately preceding frame. In this way, each of the frames will be registered in the same location relative to the aperture. The output signal may comprise a voltage-based signal, a current-based signal, or other suitable signal configured to move the actuator and thus the gate.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.